When an electrical conductor is struck by lightning, current and associated voltage waves travel in opposite directions along the lines towards terminal substations. These voltage waves are generally characterized by steep fronts and high amplitudes. The amplitude of the impinging wave is eventually limited by a lightning arrester. The steepness of the impinging wave on the other hand is practically determined by the corona characteristics of the line and the travel distance from the strike point to the substation and is not influenced by the arrester.
For a given distance between the arrester and protected equipment, the maximum steepness determines the necessary protection margin above the arrester protective level. Furthermore for insulated windings a steep impulse generally leads to unfavourable voltage stress of the insulation.
It is to be noted that overvoltages in electrical transmission systems may also be caused by internal switching operations that are not caused by lightning.